(i) Field of the Invention
The present invention relates to a shutter control apparatus for a camera. More specifically, the present invention relates to a technique for correcting timing for closing a shutter blade in accordance with a variation of a power supply voltage fed from a battery.
(ii) Description of the Related Art
A shutter control apparatus executes the exposure operation of a camera by controlling an actuator for driving a shutter blade to be opened and closed. A conventional shutter control apparatus is basically made up of a drive circuit and a control circuit. Upon receiving supply of a power supply voltage from a battery, the drive circuit switches the power for opening operation and the power for closing operation to be supplied to the actuator. The control circuit sets a timing for switching to the closing operation in accordance with at least brightness information of an object in order to control the drive circuit. The power supply voltage fed from the battery suffers from the downward variation with time. In accordance with this variation, the power supplied from the drive circuit to the actuator is reduced, which delays the shutter operation. When the shutter operation is delayed, the overexposure occurs. In order to eliminate instability of the shutter operation due to such a variation of the power supply voltage, a technique for feeding the power to the drive circuit through a constant voltage circuit or a constant current circuit has been proposed, and it is disclosed in, for example, U.S. Pat. No. 4,104,666 (Japanese patent application laid-open No. 60618-1977) and U.S. Pat. No. 4,280,761 (Japanese patent application laid-open No. 58010-1986).
FIG. 11 is a block diagram showing an example of a prior art shutter control apparatus. The shutter control apparatus 100 executes the exposure operation of a camera by controlling a shutter motor 101 for driving a shutter blade (not shown) to be opened and closed, and includes a constant voltage circuit 107 in addition to a drive circuit 104 and a control circuit 105. The drive circuit 104 performs switching between the power for opening drive and the power for closing drive to be supplied to the shutter motor 101. The control circuit 105 sets a timing for switching to the closing drive in accordance with at least brightness information of an object in order to control the drive circuit 104. The constant voltage circuit 107 turns the power supply voltage fed from a battery 102 into a constant voltage to be supplied to the drive circuit 104 so that the operation of the drive circuit 104 is stabilized.
Providing the constant voltage circuit 107, however, causes another drawback. In general, a stabilized voltage (constant voltage) outputted from the constant voltage circuit 107 is set in accordance with the lowest level of the power supply voltage of the battery 102 in an allowable range. Therefore, the power supply voltage maintained at a high level and a medium level over the better part of life duration of the battery is forcibly lowered by the constant voltage circuit 107, and the power supply voltage is disadvantageously utilized in the lowest level. Thus, there is such a problem as that the utilization efficiency of the power supply voltage is poor. In addition, the circuit for stabilizing the power supply voltage has the inferior efficiency in terms of cost and space. For example, the voltage from the battery 102 fluctuates from 3.2 V at the maximum level to 2.2 V at the minimum level, and an average level is approximately 2.7 V. On the other hand, since the constant voltage circuit 107 attains stabilization of the power supply voltage with the minimum voltage of 2.2 V as a reference, the output voltage is approximately 1.6 V. The reduced amount of 2.2 Vxe2x88x921.6 V=0.6 V is a loss caused by the constant voltage circuit 107. The constant voltage of 1.6 V is supplied to the drive circuit 104, and 1.2 V which is obtained by subtracting another loss amount 0.4 V from the constant voltage is applied to the shutter motor 101. Therefore, the power supply voltage of 2.7 V is reduced to 1.2 V on the average and this obtained voltage is supplied to the actuator. A total loss is up to 2.7 Vxe2x88x921.2 V=1.5 V.
FIG. 12 is a typical block diagram showing another example of the conventional shutter control apparatus. Like reference numerals denote parts corresponding to those in the prior art illustrated in FIG. 11. The prior art shown in FIG. 11 uses the constant voltage circuit for stabilizing the power supply, whereas the prior art depicted in FIG. 12 uses the constant current circuit. As shown in the drawing, the drive circuit 104 is a bridge circuit consisting of four transistors Tr1 to Tr4. Switching these transistors by a control circuit 105 controls a direction of an electric current for energizing the shutter motor 101. Consequently, the shutter motor 101 rotates in the forward direction at the time of opening drive and rotates in the reverse direction at the time of closing drive. The constant current circuit for stabilizing the drive current supplied from the battery 102 to the drive circuit 104 is constituted by a current source I, a load resistance R, a pair of operational amplifiers A and four buffers B, and stabilizes the drive current flowing the shutter motor 101 through the drive circuit 104. In this case, assuming that the loss of each transistor is 0.2V, the loss caused by the drive circuit 104 is 2xc3x970.2 V=0.4 V and the loss caused by the constant current circuit is approximately 0.6 V. Therefore, the voltage applied to the ends of the shutter motor 101 is approximately 1.2 V, which is greatly reduced as compared with the power supply voltage of the battery 102. Incidentally, although a dynamic stabilization technique for performing the feedback control over the drive current by monitoring a rotational speed of the shutter motor has been proposed instead of static stabilization using the constant current circuit, the effect is not enough since the margin of the control voltage is small when the high-speed feedback characteristic is demanded. Such a technique is disclosed in, e.g., Japanese patent application laid-open No. 96925-1980, Japanese patent application laid-open No. 143545-1980 (U.S. Pat. No. 4,322,145), Japanese patent application laid-open No. 254027-1985 (U.S. Pat. No. 4,648,701), and U.S. Pat. No. 4,763,155 (Japanese patent No. 2608705).
In order to eliminate the above-described problems in the prior art, the following means is taken. That is, the present invention provides a shutter control apparatus for executing an exposure operation of a camera to photograph an object by controlling an actuator which drives a shutter blade to be opened and closed. The shutter control apparatus comprises a battery that supplies a power supply voltage, a detection circuit for detecting a variation occurring in the power supply voltage, a drive circuit that directly uses the power supply voltage to feed a power to the actuator, the drive circuit switching between an open power for opening the shutter blade and a closing power for closing the shutter blade, and a control circuit that controls the drive circuit by setting a timing for switching to the closing power in accordance with at least a brightness of an object, wherein the control circuit corrects the timing for switching to the closing power in accordance with the variation of the power supply voltage detected by the detection circuit.
Preferably, the control circuit operates when the power supply voltage lowers along a time for advancing the timing for switching to the closing power so as to compensate for a delay in the closing of the shutter blade due to reduction in the closing power caused by the lowering of the power supply voltage.
Preferably, the detection circuit detects the variation of the power supply voltage while the actuator is energized by the drive circuit before executing of the exposure operation.
Preferably, the control circuit controls the actuator for driving the shutter blade provided in a digital still camera.
According to the present invention, the power supply voltage fed from the battery is directly supplied to the drive circuit without using the constant voltage circuit or the constant current circuit. This prevents loss of the power supply voltage and attains improvement of the power utilization efficiency. For example, when the power supply voltage is 2.7 V which is the average level, the voltage applied to the actuator corresponds to the value obtained by subtracting the loss of 0.4 V caused by the drive circuit from the power supply voltage, and it is calculated as 2.3 V. Further, when the power supply voltage is reduced to 2.2 V which is the lowest level, the drive voltage is 2.2 Vxe2x88x920.4 V=1.8 V. On the other hand, in the prior art shutter control apparatus incorporating therein the constant voltage circuit or the constant current circuit, the net drive voltage applied to the actuator is 1.2 V irrespective of the power supply voltage of the battery. Therefore, according to the present invention, the utilization efficiency of the power supply voltage is improved 50% to 100% as compared with the prior art. When directly feeding the power to the drive circuit by the battery, however, the operation becomes unstable due to a variation in the power supply voltage. Therefore, in the present invention, stabilization is achieved by correcting the timing for switching to the closing drive in accordance with a variation in the power supply voltage detected by the detection circuit. Specifically, the correction is made so as to advance the timing for switching to the closing drive in order to cancel out the delay of the shutter operation caused due to reduction in the drive power generated when the power supply voltage suffers from the downward variation with time. With the above described measure, increase in efficiency of power supply and stabilization of the shutter control apparatus can be both achieved.